The use of steam at higher pressures and temperatures contributes to the increase of efficiency of a steam turbine. The use of steam with such a steam condition makes increased demands on the corresponding steam turbine.
Each turbine, or turbine section, which is exposed to through-flow of a working medium in the form of steam, is understood by a steam turbine in the meaning of the present application. In contrast to this, gas turbines are exposed to throughflow by gas and/or air as working medium, which, however, are subjected to entirely different temperature and pressure conditions than the steam in the case of a steam turbine. Unlike gas turbines, for example the working medium which flows to a turbine section in the case of steam turbines has the highest pressure simultaneously with the highest temperature. An open cooling system, as in the case of gas turbines, cannot be realized, therefore, without external feed.
A steam turbine customarily comprises a rotatably mounted rotor which is populated with blades and arranged inside a casing shell. When the flow space, which is formed by the casing shell, is exposed to throughflow with heated and pressurized steam, the rotor, via the blades, is set in rotation by means of the steam. The blades which are attached on the rotor are also referred to as rotor blades. Furthermore, stationary stator blades, which engage in the interspaces of the rotor blades, are customarily attached on the casing shell. A stator blade is customarily mounted at a first point along an inner side of the steam turbine casing. In this case, it is customarily part of a stator blade ring which comprises a number of stator blades which are arranged along an inner circumference on the inner side of the steam turbine casing. In this case, each stator blade points radially inwards with its blade airfoil. A stator blade ring at a point along the axial extent is also referred to as a stator blade row. A number of stator blade rows are customarily arranged one behind the other.
Cooling plays an essential role when increasing the efficiency. With the previously known cooling medium methods for cooling a steam turbine casing, a distinction is to be made between active cooling and passive cooling. With active cooling, cooling is brought about by means of a cooling medium which is fed separately to the steam turbine casing, i.e. in addition to the working medium. In contrast, passive cooling is carried out simply by means of suitable guiding or use of the working medium. Customary cooling of a steam turbine casing is limited to passive cooling. Therefore, it is known for example to flow-wash an inner casing of a steam turbine with cool, already expanded steam. However, this has the disadvantage that a temperature difference over the inner casing wall must remain limited, since otherwise with a temperature difference which is too great the inner casing would thermally deform too much. During flow-washing of the inner casing, heat dissipation certainly takes place, but the heat dissipation takes place relatively far away from the point of heat input. Heat dissipation in direct proximity to the heat input has not previously been put into effect in sufficient measure. A further passive cooling can be achieved by means of a suitable design of the expansion of the working medium in a so-called diagonal stage.
By this, however, only a very limited cooling effect upon the casing can be achieved.
The steam turbine shafts, which are rotatably mounted in the steam turbines, are thermally highly stressed during operation. The development and production of a steam turbine shaft is at the same time expensive and time-consuming. The steam turbine shafts are considered as the most highly stressed and most expensive components of a steam turbine. This applies more and more to high steam temperatures.
Sometimes, on account of the high masses of the steam turbine shafts, these are thermally sluggish which has a negative effect during a thermal load changing of a turbine-generator set. That means that the reaction of the entire steam turbine to a load change depends in a high degree upon the speed of the steam turbine shaft being able to react to thermally changed conditions. For monitoring the steam turbine shaft, as standard the temperature is monitored, which is time-consuming and costly.
One characteristic of steam turbine shafts is that these do not have an essential heat sink. Therefore, cooling of the rotor blades, which are arranged on the steam turbine shaft, proves to be difficult.
For improving the adaptation of a steam turbine shaft to a thermal stress, it is known to form this hollow in the inlet region, or to form this as a hollow shaft. These cavities as a rule are closed off and filled with air.
However, the high stresses which occur during operation, which for the most part consist of tangential stresses from the centrifugal force, act disadvantageously upon the aforementioned steam turbine hollow shafts. These stresses are about twice as high as the stresses which would occur in the case of corresponding solid shafts. This has a strong influence upon the material selection of the hollow shafts, which can lead to the hollow shafts not being suitable, or not realizable, for high steam conditions.
In gas turbine construction, it is known to construct air-cooled hollow shafts as thin-walled welded constructions. It is known inter alia to form the gas turbine shafts with disks via so-called Hirth toothing. These gas turbine shafts have a central tie-bolt for this.
However, a direct transfer of the cooling principles in gas turbines to steam turbine construction as a rule is not possible, since a steam turbine, unlike the gas turbine, is operated as a closed system. By this, it is to be understood that the working medium is located in a circuit and is not discharged into the environment. The working medium which is used in a gas turbine, which consists essentially of air and exhaust gas, is discharged into the environment after passage through the turbine unit of the gas turbine.
Steam turbines, furthermore, unlike the gas turbine, do not have a compressor unit, and, moreover, the shafts of the steam turbine are generally only radially accessible.
Steam turbines with a steam inlet temperature of approximately 600° C. were developed and constructed in the 1950s. These steam turbines have radial blading. Today's prior art in steam turbine construction comprises shaft cooling systems with a radial arrangement of the first stator blade row in the form of diagonal or governing stages. With this embodiment, however, the low cooling action of these diagonal or governing stages is disadvantageous.
In the steam turbine shafts, the piston region and inlet region are particularly thermally loaded. The region of a thrust-compensating piston is to be understood by piston region. The thrust-compensating piston acts in a steam turbine in such a way that with a force, which is created by the working medium, upon the shaft in one direction, an opposing force is developed in the opposite direction.
Cooling of a steam turbine shaft is described inter alia in EP 0 991 850 B1. In this case, a compact or high-pressure and intermediate-pressure turbine section is constructed by means of a connection in the shaft, through which a cooling medium can flow. With this, it is considered disadvantageous that a controllable bypass cannot be formed between two different expansion sections. Furthermore, problems during variable load operation are possible.
It would be desirable to form a steam turbine which is suitable for high temperatures.